While narcotic analgesics such as morphine are important drugs for the treatment of severe pain, their use is limited because of serious side effects such as tolerance, respiratory depression, and the potential for drug abuse. Thus there is considerable interest in identifying analgesics with reduced liabilities for clinical use. Kappa opioid receptor (KOPr) ligands have shown therapeutic value in the treatment of pain, and potential for the treatment of mood disorders and drug abuse as well. Therefore exploring the structural requirements for KOPr activity is important to developing KOPr ligands as potential new medications. The proposed research focuses on peptide ligands for KOPr, which have not been studied as extensively as peptides for other opioid receptors. Recent studies in our laboratories have successfully identified a number of KOPr peptide ligands with unique pharmacological profiles that offer distinct advantages over current KOPr ligands, including some with potent antinociceptive activity in vivo. Therefore the goals of the proposed research in this competitive renewal application are to continue our structural studies of key peptidic ligands with activity at KOPr and to expand these studies to include evaluating peptides for opioid activity in vivo. The long term objectives of this research are to explore the structure-activity relationships (SAR) of peptide ligands with KOPr activity and to identify peptidic KOPr ligands with desirable pharmacological profiles for use in vivo. Three specific aims will be pursued to accomplish these goals: 1) To explore the SAR of novel analogs of the dynorphins, the endogenous ligands for KOPr, 2) to explore the SAR of novel KOPr peptides that are structurally unrelated to the dynorphins, and 3) to evaluate peptide KOPr ligands in vivo using mouse models of analgesia, antinociceptive tolerance, physiological responses and place conditioning. We hypothesize that peptide agonists with mixed agonist/KOPr antagonist activity will demonstrate significant antinociception with decreased liabilities. The insights from these studies will further enhance our understanding of the roles of KOPr in various pharmacological processes and advance the development of KOPr ligands with potential for therapeutic application.